1. Field of Invention
The present invention relates in general to power converters, and more particularly, to the control circuit of switching power converters.
2. Description of Related Art
Power converters have been used to convert an unregulated power source to a regulated voltage and/or a current source. The control circuit of the power converter generates a switching signal for the regulation. The duty cycle of the switching signal is modulated in accordance with the output of the power converter. The switching frequency of the switching signal generally is self-determining in the control circuit of the power converter. Therefore, the synchronization of the switching signal is required to reduce the switching noise and EMI (electrical and magnetic interference) while more than one switching power converters are used in a system. However, in recent development, many PWM (pulse width modulation) control circuits have been proposed for power converters to save power losses, such as “PWM controller having off-time modulation for power converter” by Yang, U.S. Pat. No. 6,545,882; “PWM controller having a modulator for saving power and reducing acoustic noise” by Yang, et al, U.S. Pat. No. 6,781,356. The switching frequency of these prior arts is varied in response to the change of the load, which causes the difficult for the control circuit to synchronize the switching signal with other power converters.
The objective of the present invention is to provide a synchronous switching control circuit to synchronize the switching signal for variable switching frequency power converters.